A structure of a conventional organic EL element (refer to non-patent document 1, for example) is shown in FIG. 2. An organic EL element 100 is formed by laminating an anode 11, an organic layer 14 including a hole transport layer 12 and an emitting layer 13, and a cathode 15 on a substrate 10 in this order. One surface of the substrate 10 which is on an opposite side of a surface facing the anode 11 has contact with an atmosphere 16. When a voltage is applied, the anode 11 injects holes into the emitting layer 13 and the cathode 15 injects electrons into the emitting layer 13, and the holes and the electrons are recombined in the emitting layer 13. This recombination causes excitons to be generated, and when the excitons return to their ground state, photons are emitted and extracted outside through the anode 11 and the substrate 10.
When light propagates from a medium with a high refractive index to a medium with a low refractive index, a critical angle at an interface therebetween is determined based on the refractive index between the media in accordance with Snell's law, and light which has a higher incident angle than the critical angle is totally reflected at the interface, confined to the medium with the high refractive index, and lost as guided light.
A refractive index of each layer in the organic EL element 100 is described below. Glass is solely used for the substrate 10 from a standpoint of excellent transparency, intensity, low cost, gas barrier layer, chemical resistance, heat resistance, etc., and a refractive index of a general soda-lime glass or the like is around 1.52.
Indium Tin Oxide (ITO), which is indium oxide doped with tin oxide, or Indium Zinc Oxide (IZO) is widely used for the anode 11 due to its excellent transparency and electric conductivity. Although refractive indexes of ITO and IZO change in accordance with a composition, a film formation method, a crystal construction, or the like, ITO and IZO have extremely the high refractive indexes of approximately 1.7 to 2.3 and approximately 1.9 to 2.4, respectively.
An emitting material, an electron transporting material, a hole transporting material, or the like which is used for the organic layer 14 is a material which has a π conjugated bond system including a number of general benzene rings in its molecular structure, so that its refractive index is approximately 1.6 to 2.0.
Thus, in the organic EL element 100, a magnitude relation among the refractive indexes of the respective layers is expressed as follows: the atmosphere 16 being in contact with the substrate 10<the substrate 10<the organic layer 14<the anode 11. Accordingly, light which is obliquely outputted from an emitting source 13a of the emitting layer 13 in the organic layer 14 at a high angle is totally reflected at an interface between the anode 11 and the substrate 10 and an interface between the substrate 10 and the atmosphere 16 (indicated by a dashed arrow).
Here, the refractive indexes of the atmosphere 16, the substrate 10, the anode 11, the hole transport layer 12, and the emitting layer 13 are expressed as n16, n10, n11, n12, and n13, respectively. Moreover, incident angles of light from the emitting layer 13 to the hole transport layer 12, from the hole transport layer 12 to the anode 11, from the anode 11 to the substrate 10, and from the substrate 10 to the atmosphere 16 are expressed as θ13-12, θ12-11, θ11-10, and θ10-16, respectively, and an output angle of light from the substrate 10 to the atmosphere 16 is expressed as θ16. An equation 1 described below is formed in accordance with Snell's law.n13 sin θ13-12=n12 sin θ12-11=n11 sin θ11-10=n10 sin θ10-16=n16 sin θ16  [Equation 1]
Equations 2 to 4 described below are extracted from the above equation 1 with a focus on a relationship between the emitting layer 13 and the hole transport layer 12, the substrate 10, and the atmosphere 16 which have lower refractive indexes than that of the emitting layer 13.n13 sin θ13-12=n12 sin θ12-11  [Equation 2]n13 sin θ13-12=n10 sin θ10-16  [Equation 3]n13 sin θ13-12=n16 sin θ16  [Equation 4]
Critical angles θc12, θc10, and θc16 of the hole transport layer 12, the substrate 10, and the atmosphere 16, respectively, are obtained from equations 5 to 7 described below in accordance with the above equations 2 to 4.
                              θ                      C            ⁢                                                  ⁢            12                          =                              sin                          -              1                                ⁡                      (                                          n                12                                            n                13                                      )                                              [                  Equation          ⁢                                          ⁢          5                ]                                          θ                      C            ⁢                                                  ⁢            10                          =                              sin                          -              1                                ⁡                      (                                          n                10                                            n                13                                      )                                              [                  Equation          ⁢                                          ⁢          6                ]                                          θ                      C            ⁢                                                  ⁢            16                          =                              sin                          -              1                                ⁡                      (                                          n                16                                            n                13                                      )                                              [                  Equation          ⁢                                          ⁢          7                ]            
When n13=1.8, n12=1.6, n10=1.52, and n16=1.0, for example, are substituted into the above equations 5 to 7, the critical angles θC12, θC10, and θC16 are determined to be 63°, 58°, and 34°, respectively. Light which is outputted from the emitting source 13a of the emitting layer 13 at a higher angle than the above angles is confined to the emitting layer 13, the anode 11, or the substrate 10, which causes a light loss. Thus, light extraction efficiency in the organic EL element 100 is reduced and external quantum efficiency is thereby reduced. The light extraction efficiency indicates a ratio between photons generated in the emitting layer and photons which reach any layer or which are emitted to the atmosphere out of the photons generated in the emitting layer. The external quantum efficiency indicates a ratio of a total number of photons which reach any layer or photons which are emitted to the atmosphere to a total number of electrons recombined in the emitting layer. The external quantum efficiency is obtained by multiplying the above light extraction efficiency by an internal quantum efficiency. The internal quantum efficiency indicates a ratio of the total number of generated photons to the total number of the electrons recombined in the emitting layer.
A method for decreasing the above light loss includes lowering the refractive index n13 of the emitting layer 13 so that the critical angle is widened. The non-patent document 1 describes a technique of mixing SiO2 into the emitting layer 13 which is formed of MEH-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene]) as the above method. A refractive index of SiO2 is 1.6, lower than that of MEH-PPV, so that the refractive index of the emitting layer 13 is lowered by mixing SiO2 particles and the quantum efficiency increases 1.45-fold.
However, even when SiO2 particles which have the refractive index of 1.6 are mixed, the refractive index of the emitting layer 13 does not fall below 1.6 but remains higher than the refractive index n10=1.52 of the substrate 10 and the refractive index n16=1.0 of the atmosphere 16. As a result, a lot of light is still confined to the anode 11 and the substrate 10 and is thereby lost. Consequently, a further reduction of the refractive index of the emitting layer 13 and an enhancement of the light extraction efficiency are required.